The 2011 Infiniti M37 and M56 replace the 2010 M35 and M45. The difference in name reflects switches to a 3.7-liter V-6 and a 5.6-liter V-8, both of which provide more power, as you'd expect. Many other upgrades come with this complete redesign.

Though beautifully styled inside and outside, the redesigned 2011 Infiniti M sport sedan is frustrated by some quirks and glitches, and it's not as affordable as it used to be.

The M comes in three versions: the M37, M56 and an all-new hybrid version, the M35h. The gas-only versions are rear-wheel drive but offer optional all-wheel drive (designated by the "x" suffix). Unlike some competitors, including the BMW 5 Series and Acura TL, the M sedan doesn't offer a manual transmission. We tested the M37x and both the M56 and M56x, but did not drive the M35h.

Transmission Misbehavior

Though the car impresses, its sport-sedan billing leads me to start with my main complaint: The seven-speed automatic transmission has a chronic case of indecision. Stomp on the accelerator from a standstill, and all is well. Ditto for normal, sedate driving. But when the time comes to pass or get moving more quickly once you're already in motion, the car bogs down as the transmission hunts for a gear.

In this regard, Infiniti's earlier five-speed automatic was better. This pattern is all too familiar: Automakers make the change to higher gear counts for their automatic transmissions — for all of the undeniable benefits — and then the immediacy of the accelerator response suffers, an unacceptable and unnecessary tradeoff.

I cited similar frustrations with the competing BMW 535i's new eight-speed automatic, but the problem is more pronounced in the M37. Further, BMW's V-8 version, the 550i, performed much better, while the M56 exhibits the M37's same bad habits.

Unless you're content to take over shifting responsibility using the transmission's manual mode and gear selector (or shift paddles that come with the optional Sport Package), the best mitigation is the Sport mode on the center console's Infiniti Drive Mode Select knob. Along with making the accelerator pedal more sensitive, it makes the transmission hold onto lower gears longer, cutting down on the delay. But that also serves to decrease the M56's mileage, an already unimpressive 16/23 mpg city/highway with all-wheel drive or 16/25 mpg without. The M37x is rated 17/24 mpg, and the M37 is 18/26 mpg. The M35h hybrid gets an estimated 27/32 mpg.

To help drivers maximize mileage, IDMS also includes an Eco mode, along with Normal and Snow. Eco does the opposite of Sport. I found it most attractive when using the adaptive cruise control, because it kept the car from surging forward aggressively if the car ahead of me moved out of the lane. It also eliminated the system's tendency to slow or brake too aggressively when the lead car slowed. Unfortunately, on a highway trip in winter, the system failed completely because the bumper-mounted laser sensor became caked with ice, prompting a "Clean Sensor" alert on the instrument panel. This also meant the optional Forward Collision Warning had become disabled. When the sensor fails, you don't simply lose the adaptive operation; the cruise control quits altogether.

This might have been a rare and unlikely occurrence, but it's one I've never experienced with the more common radar-based active cruise control most brands employ.

Take the V-6

In what is becoming a common refrain, I'm pleased to report that you don't need a V-8 in this car. Between the M37's robust 330-horsepower V-6 and seven-speed transmission, it's more than quick enough, even for a luxury buyer. If you still want more power, perhaps to offset the added weight of all-wheel drive, the V-8 provides exhilarating performance and wonderful exhaust sound when running full-tilt.

Though the all-wheel drive helps get all the power to the road, the M56x isn't the best choice for performance driving. It's the most nose-heavy of all the variants, and you feel it when going into turns, even though the system sends a majority of power to the rear wheels when you pour on the power. In my experience, the M37x was a little more disappointing because it seemed like the front and rear axles weren't on speaking terms, resulting in repeated fore/aft weight shifts. If you want top performance, stick with the rear-wheel drive and the optional Sport Package with either engine.

The Sport Package adds 20-inch wheels with summer performance tires, a firm sport suspension, four-wheel active steering, upgraded brakes, steering-wheel shift paddles and some interior upgrades. On the M56, it also includes a bevy of non-performance features. As I said of the smaller Infiniti G37, the x versions are mainly suited for inclement weather, which they handle ably.

The M sedan handles well overall; I prefer it to the Mercedes E-Class. The new generation is softer in some ways than the original M, and while some people think this took away from the previous generation's drivability, the majority seem to experience it as refinement rather than isolation. Despite the increased comfort, there's still plenty of road feel, and body roll remains in check. The steering is sporty without being overly heavy, and the brakes are great, offering fine, linear control.

The ride quality is a nice compromise between comfort and sport, especially with the smaller wheels and higher-profile tires, but it bears noting that Infiniti doesn't offer an adaptive suspension as some competitors do, usually to even better effect.

In the Cabin

The M is appropriately roomy for a car of its size, and it has nice, comfortable seats. It has more front legroom than its main competitors and is otherwise on par in terms of interior dimensions. Two adults can find adequate room in the backseat, though the center floor hump is high and would make a third passenger unhappy — all too common in this class.

All of our editors raved about the new cabin, which combines contemporary design with some traditional, high-quality materials. Proper buttons replace the rubbery pads on the steering wheel. My sole aesthetic complaint is that Infiniti abandoned the semi-gloss wood trim I liked on the previous generation. Even worse, the optional "silver-powdered white ash" wood trim, with its glistening finish, doesn't work for me at all. One of our staffers said the silvery sheen made it look like meat that had gone bad — an observation as astute as it is unsettling.

Ergo No-Go

The M has its share of ergonomic foibles. There are too many buttons down by the driver's left knee, out of sight, including one for the optional heated steering wheel. Do you want to accidentally turn off a safety feature like the electronic stability system or blind spot warning system because you can't see what you're pushing? The power seat controls are difficult to reach when the door is closed, though the driver's memory buttons are up high on the doors, so once those settings are stored, that might be a non-issue for an owner.

The heated and/or cooled seats are controlled by knobs on the center console whose tiny indicator lights are invisible during daylight.

Finally, the optional navigation system has a touch-screen, which I always appreciate for inputting addresses and such, but the display in the center of the dashboard is positioned more for visibility than for reachability. Better for use while driving, the controller knob is only slightly closer; it would be better positioned low on the console where the driver's hand comes to rest, but that's where the IDMS knob is.

Some of the options also annoy. The Eco Pedal, an enhancement to the Eco mode, makes the accelerator resist when you push on it, ostensibly to remind you to drive conservatively. There are two selectable levels and an Off option, which I dove for like a sea lion after a squid.

Also offered is a full complement of available advanced safety features you don't need if you drive responsibly (blind spot warning, lane departure warning, keeping monkeys as pets warning … .). I defeated these, too. Ditto for the optional "Forest Air system with Advanced Auto Recirculation, Breeze Mode, Plasmacluster Air Purifier and Grape Polyphenol filter," which I would mock, but some fruit hangs too low, even for me. Most occupant reactions amounted to questioning if it were doing anything or finding it as annoying as an oscillating fan.

Taken together, all of these features make the M sedan seem like quite the nag. I'd like to see Infiniti provide more control over the features that matter and provide fewer features we're compelled to control — or disable entirely. Having said that, I commend the M sedan for retaining the tried-and-true mechanical PRND shifter and a turn -signal stalk that stays where you put it until it's done its job. In these regards, BMW has left the reservation … and then nuked it from orbit.

Safety

The Infiniti M sedan's airbags include the frontal pair, front-seat-mounted side-impact airbags and side curtains that protect front and rear occupants. Antilock brakes and an electronic stability system are standard.

In addition to the aforementioned active-safety features, the M includes an effective backup camera, but the company reserves its excellent Around View Monitor — which gives an amazing overhead view of the vehicle — for its SUVs. BMW offers a similar system on its cars.

M Sedan in the Market

Infiniti's sport sedans have succeeded as a low-cost alternative to BMWs, though the advantage isn't what it once was. In 2010, before the redesign, the M35 six-cylinder version was merely $150 cheaper than BMW's 528i, but it was almost $5,500 more affordable than the comparably powered 535i. For 2011, the gap dropped to around $3,000. Standard features did and still do favor the Infiniti, though, most notably its parking assist, keyless access and standard leather seats versus the 535i's vinyl.

Things even out a bit with the V-8s, with the M56 saving between $1,550 and $1,750 over the 550i, depending on the driveline. The less-powerful M45 cost $8,450 less than the 550i in 2010.

Now more powerful than the 550i, the M56 is a worthy competitor in many ways, but the transmission is a shortcoming that sullies the M sedan's claim to sport stardom.

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Comments our editors find particularly useful or relevant are displayed in Top Comments, as are comments by users with these badges: . Replies to those posts appear here, as well as posts by staff writers.